The multi-child family people are already retired, though, and long-retired at that. Them dying out means a shrinking population but also lower pension costs with no lost labor.

The working population has been steady-state at 1-child for 40 years followed by 2-child for the last few years.

I suppose there could be some demand loss due to fewer people demanding things, but.. there's a lot of room for additional demand in China. Plenty of people where running water is a last-20-years nice new thing.